Electronic circuits, such as integrated circuits (ICs), are used in a variety of products, from automobiles to microwaves to personal computers. Designing and fabricating IC devices typically involves many steps, sometimes referred to as the “design flow.” The particular steps of a design flow often are dependent upon the type of the circuit, its complexity, the design team, and the circuit fabricator or foundry that will manufacture the circuit. Typically, software and hardware “tools” verify the design at various stages of the design flow by running software simulators and/or hardware emulators. These steps aid in the discovery of errors in the design, and allow the designers and engineers to correct or otherwise improve the design.
Several steps are common to most design flows. Initially, the specification for a new circuit is transformed into a logical design, sometimes referred to as a register transfer level (RTL) description of the circuit. With this logical design, the circuit is described in terms of both the exchange of signals between hardware registers and the logical operations that are performed on those signals. The logical design typically employs a Hardware Design Language (HDL), such as the Very high speed integrated circuit Hardware Design Language (VHDL). The logic of the circuit is then analyzed, to confirm that it will accurately perform the functions desired for the circuit. This analysis is sometimes referred to as “functional verification.”
After the accuracy of the logical design is confirmed, it is converted into a device design by synthesis software. The device design, which is typically in the form of a schematic or netlist, describes the specific electronic devices (such as transistors, resistors, and capacitors) that will be used in the circuit, along with their interconnections. This device design generally corresponds to the level of representation displayed in conventional circuit diagrams. The relationships between the electronic devices are then analyzed, to confirm that the circuit described by the device design will correctly perform the desired functions. This analysis is sometimes referred to as “formal verification.” Additionally, preliminary timing estimates for portions of the circuit are often made at this stage, using an assumed characteristic speed for each device, and incorporated into the verification process.
Once the components and their interconnections are established, the design is again transformed, this time into a physical design that describes specific geometric elements. This type of design often is referred to as a “layout” design. The geometric elements, which typically are polygons, define the shapes that will be created in various layers of material to manufacture the circuit. Typically, a designer will select groups of geometric elements representing IC components (e.g., contacts, channels, gates, etc.) and place them in a design area. These groups of geometric elements may be custom designed, selected from a library of previously-created designs, or some combination of both. Lines are then routed between the geometric elements, which will form the wiring used to interconnect the electronic devices. Layout tools (often referred to as “place and route” tools), such as Mentor Graphics' IC Station or Cadence's Virtuoso, are commonly used for both of these tasks.
Circuit layout descriptions can be provided in many different formats. The Graphic Data System II (GDSII) format is a popular format for transferring and archiving two-dimensional (2D) graphical circuit layout data. Among other features, it contains a hierarchy of structures, each structure containing layout elements (e.g., polygons, paths or poly-lines, circles and textboxes). Other formats include an open source format named Open Access, Milkyway by Synopsys, Inc., EDDM by Mentor Graphics, Inc., and the more recent Open Artwork System Interchange Standard (OASIS) proposed by Semiconductor Equipment and Materials International (SEMI). These various industry formats are used to define the geometrical information in layout designs that are employed to manufacture integrated circuits. Once the design is finalized, the layout portion of the design can be used by fabrication tools to manufacture the circuit using a photolithographic process.
There are many different fabrication processes for manufacturing a circuit, but most processes include a series of steps that deposit layers of different materials on a substrate, expose specific portions of each layer to radiation, and then etch the exposed (or non-exposed) portions of the layer away. For example, a simple semiconductor device component could be manufactured by the following steps. First, a positive type epitaxial layer is grown on a silicon substrate through chemical vapor deposition. Next, a nitride layer is deposited over the epitaxial layer. Then specific areas of the nitride layer are exposed to radiation, and the exposed areas are etched away, leaving behind exposed areas on the epitaxial layer, (i.e., areas no longer covered by the nitride layer). The exposed areas then are subjected to a diffusion or ion implantation process, causing dopants, for example phosphorus, to enter the exposed epitaxial layer and form charged wells. This process of depositing layers of material on the substrate or subsequent material layers, and then exposing specific patterns to radiation, etching, and dopants or other diffusion materials, is repeated a number of times, allowing the different physical layers of the circuit to be manufactured.
Each time that a layer of material is exposed to radiation, a mask must be created to expose only the desired areas to the radiation, and to protect the other areas from exposure. The mask is created from circuit layout data. That is, the geometric elements described in a layout design define the relative locations or areas of the circuit that will be exposed to radiation through the mask. A mask or reticle writing tool is used to create the mask based upon the layout design, after which the mask can be used in a photolithographic process.
As designers and manufacturers continue to increase the number of circuit components in a given area and/or shrink the size of circuit components, the shapes reproduced on the substrate (and thus the shapes in the mask) become smaller and are placed closer together. This reduction in feature size increases the difficulty of faithfully reproducing the image intended by the layout design onto the substrate. Adding to the difficulty associated with increasingly smaller feature size is the diffractive effects of light. These effects often result in defects where the intended or “target” image is not accurately “printed” onto the substrate during the photolithographic process, creating flaws in the manufactured device. One type of flaw is the proximity effect. The proximity effect refers to variations in the linewidth of a feature (or a shape for a 2D pattern) as a function of the proximity of other nearby features. The simplest example of a proximity effect is the difference in printed linewidth between an isolated line and a line in a dense array of equal lines and spaces.
To address the problem, one or more resolution enhancement techniques are often employed to improve the resolution of the image that the mask forms on the substrate during the photolithographic process. Examples of various resolution enhancement techniques are discussed in “Resolution Enhancement Technology: The Past, the Present, and Extensions for the Future,” Frank M. Schellenberg, Optical Microlithography XVII, edited by Bruce W. Smith, Proceedings of SPIE Vol. 5377, which article is incorporated entirely herein by reference. One of these techniques, “optical proximity correction” or “optical process correction” (OPC), adjusts the amplitude of the light transmitted through a lithographic mask by modifying the layout design data employed to create the mask. For example, edges in the layout design may be adjusted to make certain portions of the geometric elements larger or smaller, in accordance with how much additional light exposure (or lack of exposure) is desired at certain points on the substrate. When these adjustments are appropriately calibrated, proximity effects are reduced and overall pattern fidelity is greatly improved.
In addition to reducing proximity effects, process variations (or process errors) must also be considered to achieve high lithographic quality. There are a very large number of potential process errors in the semiconductor fabrication plant, from variations in the wafer filmstack to batch-to-batch variations in resist properties, from scanner stage vibrations to PEB (post-exposure bake) hot plate temperature nonuniformities. Most errors in the fabrication plant may be classified into two basic categories: errors that behave like dose errors (referred to as dose errors hereafter) and errors that behave like focus errors (referred to as focus errors hereafter). Engineers sometimes define a third category of process errors, called masking sizing errors. Mask sizing errors arise from the fact that masks are exposed progressively in small sections and process conditions may change during the exposure process. Any of the three process error categories can affect lithographic quality.
The term “process window” is often used to characterize how the lithographic process responds to changes in focus and doses. A process window may be defined as a window made by plotting contours that correspond to various specification limits as a function of process variations. One common process window, called the CD process window, is a contour plot of the high and low critical dimension (CD) specifications as a function of dose and focus variations. Here, the term CD, also called the linewidth or feature width, refers to the size (width) of a feature printed in resist, measured at a specific height above the substrate. The contour plot form of data visualization is useful for establishing the limits of exposure and focus that allow the printed image to meet certain specifications. Another form often used for visualizing the impact of process variations is the process variability (PV) band. The PV band defines the region of edge placement uncertainty when the layout design is subject to process variations. It is plotted as a band along the edges of layout features. The larger the process variation, the wider the PV band. The PV band may be derived using a process window simulation which simulates process variations.
A layout design may have regions with printability problems that are caused by process variations unavoidable in the fabrication plant. These regions are lithography hotspots or litho hotspots. Litho hotspots may be corrected by modifying layout designs in such a manner as to cause a change in the final printed contours. For example, pinching hotspots require an increase of the width of a printed contour subject to pinching while bridging hotspots requires an increase of the spacing between two printed contours subject to bridging. This layout modification procedure is often referred to as retargeting as the original drawn target of layout features is changed. Retargeting can be performed by either designers or manufacturers. As for the latter, retargeting is usually performed along with OPC and with the help of process window simulations. OPC and process window simulations are computationally demanding. It is thus desirable to develop efficient methods for retargeting.